Electrochemical storage of thermal energy

ABSTRACT

The invention relates to an energy conversion and storage system and a method wherein a battery catholyte composition (e.g., following or during a discharge process) is decomposed by heating to produce a decomposed catholyte and separate an anolyte component therefrom, thereby enabling use of the anolyte component and a remainder of the decomposed catholyte in anodic and cathodic half-cells of an electrochemical energy storage device.

TECHNOLOGICAL FIELD

The present application provides techniques for conversion and storageof thermal energy in a battery.

BACKGROUND

A crucial issue for concentrated solar power (CSP) technologies today isproviding energy storage means capable of accumulating dispatchablepower by storing solar energy. It is recognized today that furthercommercial deployment of CSP on a large scale depends on increasing thesolar capacity factor (annual contribution of solar electricity) andproper dispatchability to cope with the intermittent nature of thisrenewable power resource.

Thermal storage concepts proposed and developed heretofore are mainlybased on final production of steam in the discharge of the storage andoperating a Rankine cycle. These solutions typically result in 20-25%peak conversion efficiency from solar input to electricity and with aconversion efficiency of about 15-20% on annual average. A main reasonfor these rather low efficiency figures (resulting in economicalcompetitiveness deficiency) is due to the fact that in addition toinevitable optical and thermal losses incurring in the solar field, thereceiver and the storage subsystems, which amount to about 40-45% of thesolar input (losses), a 35-40% penalty factor also exists in thepower-block unit in the process of converting heat energy intoelectricity. This results in a final efficiency of about 15-20% fromsolar to electricity.

Additionally, the power blocks currently used in the CSP plants costsabout 15-25% of the total initial investment (depending on the size ofthe storage unit), and require large amounts of water (for cooling thecondenser of the steam cycle), which are scarce and costly in aridareas, wherein solar plants are customarily built.

A thermally regenerated fuel cell utilizing a solid electrolyte isdescribed in U.S. Pat. No. 3,404,036. In this patent the energy storagedevice utilized comprises a solid reaction zone separator positionedbetween anodic and cathodic reaction zones which are in fluid connectionwith a decomposition chamber, wherein the solid reaction zone separatorcomprises a solid crystalline electrolyte.

GENERAL DESCRIPTION

The present invention provides energy storage techniques allowingconversion of heat energy into electrochemical energy that is stored inan electrochemical cell. The techniques disclosed herein preclude theuse of a turbine to convert heat to electricity as currently donenowadays in conventional solar plants, and the electricity is directlyobtained from the electrochemical cell during its discharge cycle.Moreover, the energy storage techniques disclosed herein preclude theuse of electric power generators (e.g., turbines, wind turbines,photovoltaic panels) which are often used to recharge electrochemicalcells by applying electrical power to the cells' electrode terminals.

The inventor of the present invention developed new efficient techniquesusable to convert thermal energy into electrochemical energy, and forstoring the same in energy storage devices (electrochemicalcell/rechargeable batteries) based on sodium-sulfur battery/celltechnology. Typically, electrochemical cells are recharged nowadays byconnecting electric power sources to the electrodes of the cells toreverse a chemical reaction occurring between catholyte and anolytecomponents of the cell during normal discharge operation.

The energy conversion and storage techniques disclosed herein arefundamentally different from conventional electro-chemical storagetechniques used nowadays mainly in that a high temperature (e.g., about1500-1700° C.) thermal energy source (e.g., concentrated solarradiation) is utilized for thermo-chemically charging theelectrochemical cells, rather than the conventional electrical chargingof these cells e.g., by photovoltaic (PV) cells or wind power resources.More particularly, the present invention provides techniques forthermo-chemically recharging the electrochemical cells by heating acatholyte composition received from the electrochemical cells todecomposition thereof, separating from the obtained decomposition ananolyte component and a catholyte component, and transferring theseparated components to the electrochemical cells, directly or throughintermediate storage tanks, for use in their discharge cycles.

The techniques described herein may be used to obtain a total efficiencyof about 50% in the conversion from solar thermal energy to electricalenergy (i.e., at least twice the efficiency of currently availablesolutions), which may result in substantial economical impact on CSPtechnologies. Additionally, by using the techniques disclosed in thisapplication, significant portions of the initial investment costsinvolved in the power block subsystem of a regular CSP plant may besaved.

In possible embodiments the catholyte component comprises Sulfur and theanolyte component comprises Sodium. For example, and without beinglimiting, the electrochemical cell may be a type of Sodium-Sulfur (Na—S)cell which produces sodium polysulfide (the catholyte composition)during its discharge process.

Some embodiments of the present invention employ electrochemical cellsof the Na—S type, as electrochemical energy storage devices. To thisend, the term anolyte is used herein to generally refer to the Na partof these electrochemical cells as residing in an anodic half-cell of anelectrochemical cell, where the sodium anolyte is converted to sodiumions, and the term catholyte is used to generally refer to the sulfurpart of the cell as material residing in a cathodic half-cell of theelectrochemical cells. During the course of the discharge cycle thecomposition of the catholyte is changed as the sulfur reacts with thesodium and the sodium polysulfide content in the catholyte is increased.

In some embodiments when the discharge cycle is completed (e.g., whenthe concentration of the sodium polysulfide in the catholyte is about60-90%, for example, depending on the storage dispatchabilitymanagement) the catholyte, containing mainly sodium polysulfide (withremaining sulfur) is pumped from the cathodic half-cell (or from abuffer storage tank attached to it) for thermal decomposition, which issomewhat equivalent to the conventional electrical charging cycle.

The present invention in some of its embodiments provides a system andmethod for converting and storing energy received from thermal energyresources in electrochemical energy storage devices (also referred toherein as batteries or electrochemical cells). The energy storage devicecomprises anolyte and catholyte materials disposed in respective anodicand cathodic half-cells separated by a membrane configured to permitselective cross-transport of ion species therebetween. Morespecifically, for the Na—S battery, sodium ions migrate from the anolytepart during the discharge phase to the catholyte part containing sulfurand react with it to form sodium polysulfide (Na₂S_(x)), whichaccumulates in the catholyte during the discharge phase. Namely, thecatholyte, which before commencement of the discharge cycle containspure sulfur, turns to a mixture of mostly sodium polysulfide and someremaining sulfur. During the charging cycle, according to someembodiments of the present invention, the sodium polysulfide isdecomposed into sodium and sulfur, the sodium is returned back to theanolyte side and the sulfur to the catholyte side (e.g., through storagetanks).

In some embodiments of the present invention, thermal energy is used inrecharge cycles of the electrochemical cells to heat catholytecomposition, drawn from the cathodic half-cell of the energy storagedevice after (or during) the discharge cycle, to a decompositiontemperature thereof. The decomposed catholyte is cooled to separate ananolyte component therefrom, and then a remainder of the decomposedcatholyte is condensed to separate the catholyte component therefrom.The extracted anolyte and catholyte components may be transferred intorespective anodic and cathodic half-cells of the electrochemical cell tocomplete the recharge cycle.

The extracted anolyte and catholyte components may be cooled to aworking temperature of the electrochemical cell (e.g., 320° C. to 380°C.) before they are transferred into their respective half-cells. Forexample, and without being limiting, the extracted anolyte and catholytecomponents may be stored in respective catholyte and anolyte vesselsconfigured to receive and store these extractions and cool them to theworking temperature of the electrochemical cell.

The present disclosure provides, in some embodiments, an energyconversion system comprising a heating apparatus configured to heat acatholyte composition to a decomposition temperature thereof, and aseparator configured to cool (e.g., to about 460-500° C.) and separateanolyte and catholyte components from the decomposed catholyte to beused in respective anolyte and catholyte half-cells of anelectrochemical energy storage device. In some embodiments the separatoris configured and operable to separate the anolyte component underapplication of vacuum conditions of about 2 to 8 mbar (absolute,corresponding to the vapor pressure of the condensed liquid sodium atthe operating temperature of the separator) to the catholytecomposition.

The energy storage device typically includes in the anodic half-cell anegative terminal (e.g., made of stainless steel), in the cathodichalf-cell a positive terminal (e.g., made of stainless steel), and amembrane e.g., made of sodium β-alumina (NaAl₁₁0₁₇) which separatesbetween the half-cells and which is configured to permit selectivemigration of ionic species from the anodic half-cell to the cathodichalf-cell (ions of sodium which migrate and react with the sulfur toform sodium polysulfide).

The heating apparatus used to decompose the catholyte composition in thecharging cycle may utilize concentrated solar energy as a heat source.For example and without being limiting, in possible embodiments theheating apparatus comprises a solar tower having a solar reactorconfigured to receive thermal energy from a solar concentrator and heatthe catholyte composition after (or during) the discharge cycle to itsdecomposition temperature (e.g., 1400 to 1900° C.), thereby obtaining agaseous mixture comprising vaporized anolyte and catholyte components(also referred to herein as vapor decomposition). In some embodimentsthe catholyte composition is heated to its decomposition temperatureunder application of vacuum conditions of about 0.01 to 1 bar (absolute)to the decomposed catholyte.

The system may further include a pump for streaming catholytecomposition from the cathodic half-cell of the energy storage device tothe heating device. Specifically, in the Na—S battery the sodiumpolysulfide, Na₂S_(x), is streamed from the cathodic half cell to thesolar reactor where it decomposes to its sodium, Na, and sulfur, S,electrolyte components. The vapor decomposition is discharged from thesolar reactor and then cooled, separated and returned to the respectivehalf cells and thus charge the battery.

In possible embodiments the separator includes a separator tankconfigured to receive a stream of the vapor decomposition from theheating device and separate said decomposed catholyte to the anolytecomponent and the remainder of the decomposed catholyte. The separatortank may be in a fluid connection with the anodic half-cell fortransferring the anolyte component in a liquid phase to the anodichalf-cell. For example and without being limiting, the separator tankmay be configured to spray/sprinkle on the stream of the vapordecomposition received from the heating device a cold fluid stream(e.g., comprising cooled anolyte component and/or inert gas) to therebyquench the vapor mixture and condense the anolyte component from thevapor decomposition into a liquefied state.

The separator may include a condenser configured to receive remaindersof the vapor decomposition in a gas phase via a vapor outlet of theseparator tank, and to extract/separate a catholyte component from saidvapors remainders. For example and without being limiting, the condensermay be configured and operable to cool the remainder of the decomposedcatholyte to a working temperature of the electrochemical energy storagedevice. The condenser may be in a fluid connection with the cathodichalf-cell for transferring the separated catholyte component in a liquidphase to the cathodic half-cell. The separator tank may further includea fluid outlet for removing the liquefied anolyte from the separator anda gas outlet for removing the remainder of the decomposed cathoytemixture.

The system may include anolyte and catholyte vessels configured torespectively receive from the separator the extracted anolyte andcatholyte components, bring them to a working temperature of the energystorage device, and thereafter transfer them to respective anodic andcathodic half-cells in the energy storage device. The anolyte vessel maybe configured as a circulating tank that receives the extracted cooled,liquefied anolyte from the fluid outlet of the separator tank, andstreams the liquefied anolyte through a circulating line connected to aspraying/sprinkling inlet of the separator tank. The spraying/sprinklinginlet is configured to spray the circulated anolyte component on thevapor decomposition introduced into the separator tank to thereby quenchand cool them to condense more anolyte.

Additionally or alternatively, a stream of cold inert gas may beinjected into the separator tank and mixed with the vapor decompositionintroduced thereinto. In a similar fashion, the cooled inert gasinjected into the separator tank separates an anolyte component from thevapors which is removed from the separator tank through the fluidoutlet. A gaseous mixture including a remainder of the vapordecomposition and inert gas is removed from the separator tank throughthe vapor outlet to the condenser, wherein the catholyte component iscooled, condensed and streamed to the catholyte vessel. The catholytevessel may include a gas outlet used for removing the inert gastherefrom and recycle it back for use in the cold inert gas streaminjected inside the separator tank.

Optionally, or additionally, the separator tank may include a heatexchanger coil through which cold fluid (e.g., water) may flow. The heatexchanger coil is configured to extract heat from the vapordecomposition introduced into the separator tank, to quench it andseparate therefrom an anolyte component in a liquefied state.

The present disclosure also provides a method of converting thermalenergy into electrochemical energy. The method comprises receiving astream of catholyte composition in a fluid phase (e.g., from anelectrochemical cell, after or during a discharge cycle thereof),heating the received catholyte composition to a predeterminedtemperature being a decomposition temperature of the catholytecomposition, thereby producing a gaseous mixture comprising vaporizedanolyte and catholyte components, cooling the gas mixture to separate ananolyte component from the vapors. In some embodiments the anolytecomponent is to be transferred for use in an anodic half-cell of anenergy storage device and the remainder of the decomposed catholyte isto be transferred for use in a cathodic half-cell of the electrochemicalcell. The method may comprise condensing a catholyte component from aremainder of the vapors mixture, and the catholyte component is to betransferred for use in the cathodic half-cell of the energy storagedevice.

The heating of the catholyte composition may be carried out using solarenergy heat sources, or any other suitable heat source capable forheating the catholyte composition to its decomposition temperature.

One aspect of the present invention relates to an energy conversionsystem comprising a heating apparatus configured and operable to heat acatholyte composition from an electrochemical cell to a decompositiontemperature thereof thereby producing a decomposed catholyte, and aseparator configured and operable to receive said decomposed catholyteand separate anolyte component therefrom, thereby enabling use of theanolyte component and a remainder of the decomposed catholyte in anodicand cathodic half-cells of an electrochemical energy storage device. Thesystem may comprise the electrochemical energy storage device, which maycomprise a Na—S battery, being connected to the separator.

Optionally, the heating apparatus comprises a solar energy concentratorutilizing concentrated solar energy as a heat source for heating thecatholyte composition in a solar reactor. To this end, in someembodiments the heating apparatus may comprise a solar reactor. Forexample and without being limiting, the solar reactor may be fabricatedat least in part from at least one of graphite and ceramic, or acombination thereof.

In some embodiments the separator comprises a separator tank configuredto separate the anolyte component and transfer it to the anodichalf-cell, and a condenser configured to receive the remainder of thedecomposed catholyte, separate catholyte component therefrom, andtransfer the catholyte component to the cathodic half-cell. Theseparator tank may include an inlet configured to receive a cold fluidstream and inject it into the separator tank to facilitate separation ofthe anolyte component. For example and without being limiting, thecooled fluid stream may comprise at least one of cooled anolytecomponent and cold inert gas. Alternatively, or additionally, theseparator tank may include a heat exchanger coil configured and operablefor passage of a cold fluid therethrough to facilitate separation of theanolyte component.

In some embodiments the system includes a heat rejection device (e.g.,heat exchanger) connected to the separator and configured to cool theseparated anolyte component received from the separator tank. Forexample and without being limiting, the heat rejection device may beconfigured and operable to cool the separated anolyte component to aworking temperature of the electrochemical energy storage device.

An anolyte vessel may be used in some embodiments to receive theseparated anolyte component from the separator and transfer it to theanodic half-cell. A circulating line may be used to connect between theseparator tank and the anolyte vessel for streaming portions of thecooled anolyte from the anolyte vessel to the inlet of the separatortank.

In some embodiments a catholyte vessel connected to the condenser of theseparator is used to receive the separated catholyte component from theseparator and transfer it to the cathodic half-cell. A gas recycle lineconnected to the catholyte vessel may be used for removing portions ofthe inert gas received in the catholyte vessel with the remainder of thedecomposed catholyte for use in the cold fluid stream and inject it intothe separator tank.

Another aspect of the present invention relates to a method ofconverting thermal energy into electrochemical energy, the methodcomprising receiving a catholyte composition in a fluid phase from anelectrochemical cell, the catholyte composition comprising anolyte andcatholyte components, heating (e.g., using solar energy) the catholytecomposition to a decomposition temperature thereof, thereby producing adecomposed catholyte mixture, separating the anolyte component from thedecomposed catholyte, transferring the anolyte component in a liquidstate to an electrochemical cell for use in an anodic half-cell thereof,and transferring a remainder of the decomposed catholyte to theelectrochemical cell for use in a cathodic half-cell thereof.

The method may include in some embodiments separating (condensing) acatholyte component from the remainder of the decomposed catholytemixture to thereby obtain a separated catholyte component to betransferred for use in the cathodic half-cell.

In some possible embodiments separating of the anolyte componentincludes spraying over the decomposed catholyte mixture a cold fluid(i.e., mixing the decomposed catholyte mixture with the cold fluid)comprising at least one of a cold inert gas and cooled anolytecomponent. For example and without being limiting, the separating of theanolyte component may comprise circulating a portion of the separatedanolyte component to spray a cold fluid stream thereof over thedecomposed catholyte mixture. Alternatively, or additionally, separatingof the anolyte component may comprise providing a heat exchanger, andstreaming through the heat exchanger a cold fluid to thereby cool thedecomposed catholyte.

Optionally, before transferring the anolyte component to the anodichalf-cell, the anolyte component is cooled to a working temperature ofthe electrochemical cell, thereby producing a cooled anolyte component.In some possible embodiments spraying of the cold fluid comprisesreceiving portion of the cooled anolyte component and spraying the sameover the decomposed catholyte.

Optionally, before transferring the catholyte component to the cathodichalf-cell, the catholyte component is cooled to a working temperature ofthe electrochemical cell.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, embodiments will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which like reference numerals are used to indicate correspondingparts, and in which:

FIG. 1 schematically illustrates a charge/discharge process inconventional Na—S battery cells;

FIG. 2 is a sectional view of a possible Na—S cell according to somepossible embodiments;

FIGS. 3A and 3B show sodium polysulfide decomposition states anddischarge state plots of Na—S battery cells, wherein FIG. 3A showstransits in the Na₂S_(x) and Sulfur concentrations relative totemperature changes, and FIG. 3B shows the changes in the electricalvoltage of the Na—S battery cell relative to the changes in the sodiumpolysulfide decomposition;

FIGS. 4A and 4B are thermodynamic plots of Na₂S_(x) and Sulfurequilibrium compositions at 1 and 0.001 bar, respectively;

FIG. 5 is a plot of pressure influence on the extent of sodiumpolysulfide decomposition reaction at 1500° C.;

FIG. 6 schematically illustrates electrochemical storage of thermalenergy in a battery according to some possible embodiments;

FIGS. 7A to 7C schematically illustrate various separation techniques ofthe present application for separating anolyte and catholyte componentsfrom vapor decomposition, wherein FIG. 7A demonstrates a techniquewherein a cooled liquefied anolyte component is sprayed in a separatortank to extract more anolyte components from the vapors, FIG. 7Bdemonstrates a technique wherein cold inert gas is injected into theseparator tank to quench the hot vapors and condense the anolytecomponents, and FIG. 7C demonstrates a technique utilizing heatexchanger coil to quench the hot vapors and extract the anolytecomponents; and

FIG. 8 is a flowchart illustrating a recharge process of anelectrochemical cell according to some possible embodiments.

It is noted that the embodiments exemplified in the figures are notintended to be in scale and are in diagram form to facilitate ease ofunderstanding and description.

DETAILED DESCRIPTION OF EMBODIMENTS

Recently there has been a growing interest in large-scale rechargeablebatteries to enable substantial integration of renewable energy sourcessuch as PV and wind into electrical grid systems. One such battery isknown as a sodium-sulfur (Na—S) battery 19, which is based on Na⁺conducting electrolyte membrane 17 (e.g., β-Al₂O₃ solidelectrolyte—BASE) that separates between molten sodium anolyte 11 and amolten sulfur catholyte 14, as schematically illustrated in FIG. 1.

The operation of Na—S batteries is principally based on the discoverythat a common ceramic refractory, sodium β-alumina (NaAl₁₁0₁₇), exhibitsextremely high ionic conductivity for Na ions. At about 300° C., theionic conductivity for NaAl₁₁0₁₇ approaches that of the aqueouselectrolyte, H₂S0₄, suggesting the possibility of using NaAl₁₁O₁₇ as asolid electrolyte in a high temperature electrochemical cell. Na—Sbattery technology has been commercialized in Japan since 2002, where itis largely used in utility based load-leveling or peak-shavingapplications. Na—S battery technology has been demonstrated at over 190sites in Japan. More than 270 MW of stored energy suitable for 6 hoursof daily peak shaving have been installed. The largest Na—S installationis a 34-MW power, 245-MWh energy storage unit alongside a 51 MW windfarm stabilizing the grid in Northern Japan. The demand for Na—Sbatteries as an effective means of stabilizing renewable energy outputand providing ancillary services is expanding. U.S. utilities havedeployed 9 MW for peak shaving, backup power, firming wind capacity, andother applications. Projections indicate that development of anadditional 9 MW is in progress. Several projects are also underdevelopment in Europe and Japan (updated April 2010).

Possible Na—S battery implementations are described in U.S. Pat. Nos.3,404,035 and 4,492,742. FIG. 1 demonstrates charge (designated byarrows 18 c) and discharge cycles (designated by arrows 18 d) of typicalNa-S battery cell 19. In the discharge cycle, positive Na⁺ ions 12 passfrom the anodic half-cell 11 h through the electrolyte 17, and electrons15 flow through the external electrical circuit 18 electricallyconnected to the battery terminals (19 p and 19 m). These types of cellsare typically capable of producing an electrical voltage of about 2volts. The battery process illustrated in FIG. 1 is reversible, and inthe conventional charging cycle the sodium polysulfide molecules 13 aredecomposed and caused to release the Na⁺ ions 12 back through theelectrolyte 17 to recombine as elemental sodium 11 by connectingelectrical power source 18 p to the battery terminals 19 p and 19 m.

The half-cell and overall-cell reactions can be expressed as follows:

Anode: 2Na⇄2Na⁺+2e ⁻  (1)

Cathode: xS+2Na⁺+2e ⁻⇄Na₂S_(x)  (2)

Overall cell reaction: xS+2Na⇄Na₂S_(x) (x=3, 4 or 5)  (3)

The open circuit voltage of the cell 19 at 350° C. is typically about2.075 V. The sodium-sulfur battery cell 19 usually works at temperaturesranging between 300 and 350° C., at which the sodium (Na) 11, sulfur (S)14 and the reaction product (Na₂S_(x)) 13 of sodium polysulfide occur inthe liquid state of these materials.

A sectional view of the Na—S battery cell 19 is schematicallyillustrated in FIG. 2, wherein the anodic half-cell 11 h defined by avessel made from the solid electrolyte 17 (e.g., Beta-Alumina tube) isimmersed in the composition of the cathodic half-cell 13 h.

Na—S batteries (19) exhibit high power, energy density, temperaturestability, are relatively inexpensive and are considered to be safe. Thespecific energy density of these batteries is typically about 760 W-h/kgat 350° C., nearly three times the energy density of a lead-acidbattery. These batteries also have high efficiency, of about 89-92% intotal for charge/discharge cycle. In the embodiments disclosed hereinonly the discharge half cycle is done electrochemically through thebattery membrane, and the recharge cycle is done externally by thethermal decomposition (i.e., the Na ions pass through the membrane onlyonce during the discharge cycle (e.g., demonstrated by arrowed lines 18c in FIG. 1), whereas in the conventional electrical battery rechargethey pass through the membrane also in the recharge cycles i.e., doublepass, but in the opposite direction (e.g., arrowed lines 18 d in FIG.1), therefore the expected efficiency of the battery is greater than 90%half cell on the discharge part. Na—S batteries also have pulse powercapabilities, over six times greater than their continuous dischargerating. These attributes enable the Na—S battery to be economically usedin combined power quality and peak shaving applications.

Some properties of the Na₂S_(x) composition pertaining to the techniquesof the present application will be now discussed herebelow.

The composition of the sodium polysulfide (Na₂ S_(x)) obtained at thecathodic half-cell 13 h changes with the state-of-discharge of thebattery 19. The changes in the sodium polysulfide composition areindicated by the number of Sulfur (S) atoms, i.e., designated by thevalue of x in the expression Na₂S_(x).

When in a high state-of-discharge, in which the electromotive force(i.e., cell EMF/voltage) is essentially constant (2.075 V), and at 300°C., the cathodic half-cell 13 h consists of co-existing phases of bothSulfur and Na₂S₅ (up to 60% state-of-discharge). As the cell is furtherdischarged the Sulfur atoms combine with the Na atoms to form singlephase region with x<5. The electromotive force essentially linearlydecreases as “x” decreases. At ˜80% state-of-discharge x=4 and at fulldischarge x=3 (and the cell voltage is reduced to 1.74 V). Thedependency of the Na—S cell EMF on the state of the sodium polysulfidecomposition state, and the dependency of the sodium polysulfidecomposition state on the cell temperature is illustrated in FIGS. 3A and3B.

The Na₂S_(x) composition properties discussed above are harnessed invarious thermo-chemical charging techniques of the present application.Possible embodiments for thermo-chemical charging of Na—S cellsutilizing concentrated solar energy as a heat source are explainedbelow.

Instead of charging the Na—S battery with external source of electricity(18 p in FIG. 1), as typically done today to decompose the Na₂S_(x) (13)compound back to its Na (11) and Sulfur (14) ingredients, thedecomposition process according to some embodiments of the presentinvention is carried out thermally employing a suitable heat source,such as concentrated solar energy, for example. It is noted that acombined charging of the battery by electricity from a renewableresource like wind or PV and thermal may be carried out in some possibleembodiments i.e., the recharging cycle includes both thermal rechargingand the conventional electrical recharging.

The typical reaction in such process is as follows:

Na₂S₄→2Na+4S ΔH_(25C)=411.4 kJ/mol; ΔH_(1900C)=428.5 kJ/mol

Thermodynamics show that the compounds Na₂S₄ or Na₂S₃, are similarlyfully decomposed at about 1900° C. and 1 bar pressure, as seen in FIG.4A, and at about 1400° C. and 0.001 bar absolute pressure, as seen inFIG. 4B. The decomposed products are gaseous mixtures of Sulfur andSodium vapors that need to be quenched to below the boiling point of Na(e.g., 883° C. at atmospheric pressure) where the Na atoms can beseparated in a liquid state, while the Sulfur atoms remains in theirgaseous phase until cooled to about 444.6° C.

Accordingly, in possible embodiments of the present disclosure, thecharging cycle of the Na—S battery 19 comprises heating at least aportion of the composition from the cathodic half-cell 13 h to decomposethe sodium polysulfide products (13) into its Na (11) and S (14)compounds. Separation of the decomposed products, according someembodiments, may be based on the fact that, while the decompositionvapors are being cooled, before condensation of the Sulfur atoms (14) iscommenced, most of the Sodium atoms (11) become liquefied and separated(vapor pressure of Sodium at the 445° C. is about 1 mbar abs). Theseparated sodium (11) and sulfur (14) species may be further cooled to atemperature of about 350° C. in an external heat exchanger (e.g., usingthe condenser 75 and heat exchanger 71 shown in FIGS. 7A to 7C), atwhich stage they are respectively returned in liquid state to the anodichalf-cell (11 h) and cathodic half-cell (13 h) of the Na—S battery 19 atits working temperature i.e., 350° C.

The influence of the pressure on the decomposition reaction of thesodium polysulfide (13) at 1500° C. is illustrated in FIG. 5. As seen inFIG. 5 at 1500° C. and about 50 millibar absolute pressure the Sodiumpolysulfide (13) is fully decomposed, compared to about 1900° C.required at a pressure of 1 bar (see, FIG. 4A).

A system 60 for thermo-electrochemical storage of solar energy accordingto some possible embodiments of the present invention is schematicallyillustrated in FIG. 6. In this example, in the charging cycle of thebattery 19, portions of the sodium polysulfide composition 13 are pumpedfrom the cathodic half-cell 13 h of the battery 19 and heated in theheating apparatus 65 to decompose the pumped catholyte composition intoits Na (11) and S (14) components e.g., 2Na+4S. Thereafter, thedecomposed components are directed to the separator 64 wherein thedecomposed vapor mixture is separated into its Na and S components,which are then respectively directed to Sodium vessel 62 and Sulfurvessel 63 for cooling them to the working temperature of the battery 19,before they may flow to their respective half-cells.

More particularly, pump 61 may direct a stream comprising sodiumpolysulfide composition (the catholyte composition after the dischargecycle) from the cathodic half-cell 13 h to the heating apparatus 65. Thesodium polysulfide composition is heated in the solar reactor of theheating apparatus 65 to its decomposition temperature and dischargedtherefrom in a form of Na and S decomposed vapors into the separator 64.

The separator is configured to quench and extract the Na and Scomponents from the vapors, for example, in a first stage, by coolingthe vapors to a temperature of about 460-500° C., in the case ofatmospheric pressure, to liquefy the Na component and reducesubstantially its vapor pressure (e.g., to about 2-8 mbar), and in afurther stage, by extracting the S component by condensing the vaporsremained after the cooling in the first stage.

The extracted Na and S components may be transferred to respectiveSodium and Sulfur vessels, 62 and 63, for energy storage. The Na vessel62 and the S vessel 63 may be configured to cool (e.g., using a coolingjacket or by circulating through cooling heat exchanger) the separatedNa and S components to a working temperature of the battery 19 (e.g.,about 350° C.), and thereafter permit passage of the cooled Na and Scomponents to the anodic half-cell 11 h and the cathodic half-cell 13 h,respectively. The volumes of the anolyte and catholyte vessels, 62 and63, may be set to determine the size of the energy storage (19) and theamount of energy stored in the system 60. The battery 19 has a limitedstorage volume and it is mainly used as an electrochemical converterduring the discharge cycles commenced after each refill of its anodicand cathodic half-cells, 11 h and 13 h, from the anolyte and catholytevessels, 62 and 63 respectively.

The heating apparatus 65 may be implemented in various ways employingany suitable heating technique. For example, in some possibleembodiments of the present application the heating performed in theheating apparatus 65 utilizes renewable energy resources, such as, butnot limited to, concentrated solar energy (e.g., employing a solar towerand/or dish).

In some possible embodiments of the present invention the heatingapparatus 65 is configured to heat the sodium polysulfide composition 13by solar energy. For example and without being limiting, the heatingapparatus 65 may be a solar reactor (78 shown in FIGS. 7A-C) designedfor decomposing Na₂S_(x) (x=3, 4) at temperatures of about 1500-1700° C.and pressures of about 0.01-1 bar absolute. Such possible solar reactordesigns are described and illustrated in:

U.S. Pat. No. 5,947,114 (Kribus A., Karni J. and Doron P. “Central solarreceiver with a multi component working medium”).

Karni et al., “The DIAPR: A high-pressure, high-temperature solarreceiver”, J. Solar Energy Engineering 119, 74-78, 1997.

Kribus et al., “Performance of the Directly-Irradiated AnnularPressurized Receiver (DIAPR) operating at 20 bar and 1,200° C”, J. SolarEnergy Engineering 123, 10-17, 2001.

At least some elements of the solar reactor may comprise, or befabricated from, graphite. For example and without being limiting, sucha solar reactor may comprise internal elements made from, or comprisinggraphite (e.g., graphite foam) configured to replace the ceramicconfiguration used in the above-referenced examples. Graphite may bepreferable in some embodiments as it is a good light absorber, heatconductor and is chemically resistant to the Na₂S_(x), Na and Ssubstances involved in the decomposition process. However, othermaterials may be used together with, or instead of, graphite.

As discussed hereinabove, fast quenching (cooling) and separation of thesodium and sulfur vapor products received from solar tower are requiredin order to prevent recombination. Various vapor component separationtechniques applicable in the thermo-electrochemical solar energy storagetechnique of the present application are illustrated in FIGS. 7A-C.

In some possible embodiments, the mixture of sulfur and sodium vaporsare quenched by spraying cooled liquefied Sodium into the receivedvapors, as exemplified in FIG. 7A. More particularly, in this example aconcentrator 77 (e.g., solar heliostat, dish, mirror array, or suchlike)is used to concentrate sun rays 70 onto the solar reactor 78 of theheating apparatus 65. The solar reactor 78 heats the sodium polysulfidecomposition from the cathodic half cell (13 h) to about 1400-1900° C.under pressure conditions of about 0.01-1 bar, to thereby causedecomposition thereof into vaporized Sulfur and Sodium components.Vacuum pressure conditions of up to one bar absolute, corresponding tothe temperatures range of 1400-1900° C. respectively, may be maintainedby controllably maintaining the temperature in the separator 74 at about500° C. and the temperature in the condenser 75 (on the remainder of thevapors) at about 350° C. At these temperatures the condensed phases havelow vapor pressure and thus cause vacuum pumping effect over the solarreactor 78. The vapor decomposition is streamed into the separator tank74 in which it is cooled (e.g., to about 460 to 480° C.) by recycledcooled (e.g., to about 350° C.) liquefied Sodium that is sprayed intothe vapors.

Separator tank 74 may be implemented by a vessel made of stainless steelfor example, having a vapor inlet 74 v for receiving the hot vapors 78 sstreamed from the solar reactor and a spraying inlet for 74 s (e.g.,using a spraying manifold and/or nozzles/sprinkles) for spraying thecooled liquefied Sodium from the anolyte vessel 62 into the separatortank 74. The separator tank 74 further includes a top vapor outlet 74 tfor removing a remainder of the vapors, including the sulfur vapors,from the separator tank 74, and a fluid outlet 74 f for removingliquefied Sodium from the separator tank 74.

As explained in detail below, in some embodiments the condensate liquidSodium collected at the bottom of separator tank 74, and removedtherefrom via the fluid outlet 74 f to the sodium storage vessel 62, ispassed through a heat exchanger 71 to further cool the extracted anolytecomponent to a temperature of about 350° C.

In this example the sprayed cooled/liquefied Sodium quenches the gasmixture 78 s introduced into the separator tank 74 through the vaporinlet 74 v to liquefy and separate the Sodium vapors from the vaporstream 78 s, while the Sulfur component remains in gaseous phase. Aremainder of the vapors, remaining after the quenching and the Sodiumcondensing, flows from the separator tank 74 via the vapor outlet 74 tto a condenser 75 in which the Sulfur vapors are condensed by coolingthem to about 360-380° C. The liquefied Sulfur is streamed from thecondenser 75 into the Sulfur vessel 63 in which it may be stored atabout 350° C., or close to the operating temperature of the battery, orit can be further cooled (e.g., using a cooling jacket) to about150-180° C. for longer storage terms. The cooled liquefied Sulfur may bestreamed from the Sulfur vessel 63 into the cathodic half-cell 13 h ofthe battery 13, as illustrated in FIG. 6.

As demonstrated in FIG. 7A, the cooled liquefied Sodium 11 accumulatinginside the separator tank 74 is streamed through the fluid outlet 74 finto the Sodium vessel 62. The Sodium vessel 62 may be configured tocool/maintain (e.g., using external heat exchanger or a cooling jacketwhich extract and reject the heat absorbed by the sodium stream recycledthrough the separator 74) the liquefied Sodium 11 at a temperature ofabout 350° C., such that the cooled liquefied Sodium 11 may be streamedtherefrom into the anodic half-cell 11 h, as demonstrated in FIG. 6. TheSodium vessel 62 in this example is configured as both a storage and acirculating tank having a fluid inlet 62 i through which the separatedliquefied Sodium 11 is received thereinto, and a fluid outlet 62 tthrough which a stream of cooled liquefied Sodium 11 is pumped by thepump 76 through a circulating line 62 n to the spraying inlet 74 s ofthe separator 74. Portions of the cooled Sodium 11 may be transferred tothe anodic half-cell, simultaneously or intermittently, as may beneeded.

Another possible vapor component separation technique is exemplified inFIG. 7B. The solar heating and decomposition elements shown in FIG. 7Bare substantially similar, or identical, to those described hereinabovewith reference to FIG. 7A. In this non-limiting example the streamedvapors 78 s are cooled and separated in the separator 74 by injection ofa cold inert gas 72 (e.g., Helium) into the separator 74 via the inlet74 s. The stream of cold inert gas 72 introduced into the separator 74cools the vapor 78 s and thereby liquefies Sodium vapors whichaccumulate inside separator 74 in a liquid form 11. The separatedliquefied Sodium 11 is streamed via fluid outlet 74 f into the Sodiumvessel 62, and a remainder of the vapors (comprising Sulfur vapors andinert gas 72) flow via vapor outlet 74 t through the condenser 75.Condensed liquefied Sulfur and inert gas 72 flows from the condenser 75into the Sulfur vessel 63.

The separated components may be further cooled in their respectivevessels 62 and 63 to the working temperature of the battery 19 and flowto their respective half-cells. For example, and without being limiting,in some embodiments the condensate liquid Sodium collected at the bottomof separator tank 74, and removed therefrom via the fluid outlet 74 f tothe sodium storage vessel 62, is passed through a heat exchanger 71 tofurther cool the extracted anolyte component to a temperature of about350° C.

In this example the stream received from the condenser 75 comprises thecondensed Sulfur and the inert gas 72. The condensed Sulfur accumulatesat the bottom of the Sulfur vessel 63, and the inert gas 72 is removedvia gas outlet 63 p of the Sulfur vessel 63. The inert gas 72 removedvia the gas outlet 63 p may be then recycled and cooled for further usein separation of the vapors 78 s in the separator 74.

FIG. 7C schematically illustrates yet another possible technique forvapor separation according to some embodiments of the presentapplication. The solar heating and decomposition elements shown in FIG.7C are substantially similar, or identical, to those describedhereinabove with reference to FIGS. 7A and 7B. In this non-limitingexample, the vapors 78 s received in the separator 74 from the solarreactor 78 are cooled using a heat exchanger coil 73 inside theseparator tank 74. A stream of cooling fluid 73 w (e.g., air or water)is passed through the heat exchanger coil 73 to extract heat from thevapors 78 s and liquefy and separate the Sodium component 11, whichaccumulates inside the separator 74 in liquid state.

In possible embodiments water is used as the cooled fluid 73 w thatpasses through the heat exchanger coil 73. In such applications the heatabsorbed via the heat exchanger coil 73 can raise steam which can beused to operate a regular turbine and generate electricity. In this way,efficiency of the entire system may be further increased.

In some embodiments the liquefied Sodium 11 which flows via the fluidoutlet 74 f is further cooled from about 500° C. to about 350° C. (orclose to the operating temperature of the battery) by passing it throughheat exchanger 71 and therefrom into the Sodium vessel 62, while theremaining vapors are removed from the separator tank 74 via the vaporoutlet 74 t to the condenser 75. The condensed Sulfur is streamed fromthe condenser 75 into the Sulfur vessel 63. The liquefied components maybe further cooled in their respective vessels to the workingtemperatures of the battery 19 and streamed into their respectivehalf-cells.

The condenser 75 in some embodiments is implemented as a conventionalcondenser which may be made, for example, from any suitable metallicmaterial e.g., stainless steel.

The cooling and separating of the Na and S components from the vapordecomposition may be carried out using any one of the techniques of thepresent application described hereinbelow, or any combination of thesetechniques.

As exemplified above, the anolyte component extracted in the separatormay be condensed and separated as liquid at a temperature of about 500°C., and then further cooled by a heat rejection device (e.g., heatexchanger 71) to a temperature of about 350° C. and stored in theanolyte vessel 62. The heat rejection device (71) removes out of thesystem the heat required to quench the vapors. The separator 74 operatesat a temperature of about 500° C. and at more or less the same pressureas in the solar reactor 78, where the sodium polysulfide is decomposed,in order to maintain the sulfur in its gaseous phase.

The condensed anolyte (Na) may be further cooled e.g., to about 350° C.before it is recycled to the separator. The heat rejection may becarried out by external heat exchanger, wherein the extracted heat maybe utilized to operate a steam turbine and thereby generate moreelectricity. Accordingly, solar heat absorbed in the solar reactor 78 ispartly invested in chemical decomposition, while a large portion thereofis absorbed as sensible heat in the system components. The heat absorbedin the system should be rejected, and therefore in some embodiments,some heat is removed by first quenching the gaseous mixture 78 s in theseparator 74 to minimize the back reaction, and thereafter further heatis removed by both condensing the Sulfur in condenser 75 and cooling theanolyte (Na) by external heat exchanger 71 to the battery operatingtemperature.

FIG. 8 is a flowchart illustrating a process 80 for recharging anelectrochemical cell according to some possible embodiments. The processstarts in step 81 by pumping a catholyte composition from anelectrochemical cell (19). In step 82 the pumped catholyte compositionis heated to a decomposition temperature, and in step 83 an anolytecomponent is decomposed from the decomposition. Next, in step 84 theseparated anolyte is collected, and optionally cooled (e.g., to aworking temperature of the electrochemical cell, designated by dashedbox 84 c), and in step 85 transferred to an anodic half-cell of theelectrochemical cell.

A remainder of the decomposition is collected in step 86 and processedin steps 86-88 as follows; in step 87 the remainder of the decompositionundergoes catholyte separation in which a catholyte component isseparated and in step 88 transferred to a cathodic half-cell of theelectrochemical cell. Alternatively, the remainder of the decompositionobtained in step 86 may be transferred directly to the cathodichalf-cell of the electrochemical cell (indicated by dashed arrowed line)without carrying out the separation step 87. An optional cooling step(designated by dashed box 87 c) may be included to cool (e.g., to aworking temperature of the electrochemical cell) the separated catholyte(or the remainder of the decomposed catholyte) before transferring it tothe catholyte half-cell.

Some of the economical benefits associated with electrochemical storageof thermal energy in Na-S battery cells according to embodiments of thepresent application are discussed below.

The embodiments disclosed herein provide innovative and unique energystorage techniques which may be used for storage of reusable, recyclableor renewable thermal energy (e.g., solar energy) with built-inconversion efficiency of at least 90%, from storage to electricity. Animplementation of the storage system using solar energy sources mayprovide solar to electricity (DC) conversion efficiencies of about 50%,assuming about 60% efficiency of the solar process (optical and thermallosses). This is more than twice the current commercial conversion peakefficiency from solar to electricity. From an economical point of view,investment in the power block in a regular solar power plant is avoided(estimated as 20-25% of the total cost of the plant). Production ofsteam or operation of any turbine is avoided in this new concept. Theavoidance of the regular steam turbine preclude the need for watercooling, in desert areas where solar plants are typically deployed,which overcomes a significant hurdle in the implementation of solarpower technology.

In possible embodiments of the present application the DC currentproduced when discharging the battery can be converted to AC, ordirectly transmitted as high voltage DC over transmission lines.Additionally, the Na-S battery can be still shared, in parallel to thesolar thermal plant, with other renewable energy resources such as PVand wind which may be connected to a joint smart grid.

The above examples and description have of course been provided only forthe purpose of illustration, and are not intended to limit the inventionin any way. As will be appreciated by the skilled person, the inventioncan be carried out in a great variety of ways, employing more than onetechnique from those described above, all without exceeding the scope ofthe invention.

1. An energy conversion system, comprising: a heating apparatusconfigured and operable to heat a catholyte composition to adecomposition temperature thereof thereby producing a decomposedcatholyte; and a separator configured and operable to receive saiddecomposed catholyte and separate anolyte component therefrom, therebyenabling use of the anolyte component and a remainder of the decomposedcatholyte in anodic and cathodic half-cells of an electrochemical energystorage device.
 2. The energy conversion system of claim 1, comprisingthe electrochemical energy storage device connected to said separator,the electrochemical energy storage device comprising a Na—S battery. 3.The system of claim 1, wherein the heating apparatus comprises a solarenergy concentrator, thereby utilizing concentrated solar energy as aheat source for heating the catholyte composition.
 4. The system ofclaim 1 wherein the heating apparatus comprises a solar reactorfabricated at least in part from at least one of graphite and ceramic,or a combination thereof.
 5. The system of claim 1 wherein the heatingapparatus is configured and operable to heat the catholyte compositionto a temperature in the range of 1400 to 1900° C.
 6. The system of claim1 wherein the heating apparatus is configured and operable to heat thecatholyte composition under application of vacuum conditions of about0.01 to 1 bar to the catholyte composition.
 7. The system of claim 1wherein the separator is configured and operable to cool the decomposedcatholyte to thereby cause said separation.
 8. The system of claim 7,wherein the separator is configured and operable to cool the decomposedcatholyte to a temperature of about 460-500° C.
 9. The system of claim 1wherein the separator is configured and operable to separate the anolytecomponent under application of vacuum conditions lower than the vacuumconditions in the heating apparatus.
 10. The system of claim 1 whereinthe separator comprises: a separator tank configured to separate saiddecomposed catholyte to the anolyte component and the remainder of thedecomposed catholyte, said separator tank being in a fluid connectionwith the anodic half-cell thereby transferring the anolyte component ina liquid phase to said anodic half-cell; and a condenser configured toreceive the remainder of the decomposed catholyte in a gas phase, andseparate a catholyte component therefrom, the condenser being in a fluidconnection with the cathodic half-cell to thereby transfer saidseparated catholyte component in a liquid phase to the cathodichalf-cell.
 11. The system of claim 10 wherein the condenser isconfigured and operable to cool the remainder of the decomposedcatholyte to a working temperature of the electrochemical energy storagedevice.
 12. The system of claim 10, comprising an anolyte vessel forreceiving the separated anolyte component in the liquid phase, fortransferring at least a portion thereof to the anodic half-cell.
 13. Thesystem of claim 10, wherein the separator tank includes an inletconfigured to receive a cold fluid stream and inject it into saidseparator tank to facilitate separation of the anolyte component. 14.The system of claim 13, comprising a circulating line connecting betweeninlet of the separator tank and the anolyte vessel for streaming aportion of the cooled anolyte therefrom to said inlet.
 15. The system ofclaim 13, wherein said cooled fluid steam comprises a cold inert gas.16. The system of claim 10 wherein the separator tank includes a heatexchanger configured and operable for passage of a cold fluidtherethrough to facilitate separation of the anolyte component.
 17. Thesystem of claim 1, comprising a heat exchanger connected to theseparator and configured to cool the separated anolyte component flowingout of the separator.
 18. The system of claim 17 wherein said heatexchanger device is configured to cool the separated anolyte componentto a working temperature of the electrochemical energy storage device.19. The system of claim 10 comprising a catholyte vessel connected tothe condenser of said separator to receive the separated catholytecomponent flowing from the condenser for transferring it to the cathodichalf-cell.
 20. The system of claim 19, comprising a gas recycle lineconnected to the catholyte vessel for removing portions of the inert gasfrom the remainder of the decomposed catholyte and injecting the inertgas back into the separator tank for use in a cold fluid streamfacilitating the separation.
 21. The system of claim 19, wherein thecatholyte and anolyte vessels are configured and operable to cool andmaintain the catholyte and anolyte components respectively in a workingtemperature of the electrochemical energy storage device.
 22. A methodof converting thermal energy into electrochemical energy, the methodcomprising: receiving a catholyte composition in a fluid phase from anelectrochemical cell after or during discharging of said cell, heatingsaid received catholyte composition to a predetermined temperature beinga decomposition temperature of the catholyte composition, therebyproducing a decomposed catholyte mixture in a gas phase; separating ananolyte component in a liquid phase from the decomposed catholytemixture, thereby enabling transferring said anolyte component to anelectrochemical cell for use in an anodic half-cell thereof, andtransferring the remainder of said decomposed catholyte for reuse in acathodic half-cell thereof of an electrochemical cell.
 23. The method ofclaim 22, comprising condensing the remainder of the decomposedcatholyte mixture for separating a catholyte component from the mixturethereby obtaining a separated catholyte component to be transferred foruse in the cathodic half-cell.
 24. The method of claim 22, wherein theseparating of the anolyte component comprises spraying over thedecomposed catholyte mixture, a cold fluid stream.
 25. The method ofclaim 24, wherein said cold fluid stream comprises at least a cold inertgas.
 26. The method of claim 24, wherein the separating of the anolytecomponent comprises circulating a portion of the separated anolytecomponent to spray a cold fluid stream thereof over the decomposedcatholyte mixture.
 27. The method of claim 22, wherein the separating ofthe anolyte component comprises streaming a cold fluid through a heatexchanger thereby cooling the decomposed catholyte mixture.
 28. Themethod of claim 22, comprising cooling the separated anolyte componentto a working temperature of the electrochemical cell, thereby producinga cooled anolyte component to be transferred to the anodic half-cell.29. The method of claim 23, wherein said condensing comprises coolingthe remainder of catholyte mixture to a working temperature of theelectrochemical cell.
 30. The method of claim 22, wherein the heating ofthe catholyte composition comprises is carried out using concentratingsolar energy to heat a solar reactor containing said catholytecomposition.